It doesn't count all of their total tour revenue, notice how on Britney's they only note it's based off of $21,000,000. So Rihanna made much more.
Aside from the tour, which I think still accounts a huge percentage of Rihanna's earnings for the year, one would think her massive single/album sales would compensate for the rest. She's practically ubiquitous in the same level as Gaga and Perry but her total earning doesn't translate much quite well in proportion to her popularity.
8 Celine Dion
$14,261,515 She grossed $40.1 million from 57 performances
Though Dion's last studio album was in 2007 she sold 331,000 albums in 2011
The singer also sold 956,000 digital tracks last year.